Ball Screw Drive, Ball guide

Ball Screw Drive, Ball Guide

Ball Screw Drive

A ball screw is made up of a rotating screw and a moving ball nut. The ball nut is attached to the carriage of the unit. It does not have a normal thread, instead balls circulate inside the nut making it work as an efficient ball bearing that travels along the screw. Ball screws come in a large variety of leads, diameters and tolerance classes. The tolerance class (T3, T5, T7 or T9) indicates the lead tolerance of the screw. The lower the number, the higher the tolerance. High load capability and high accuracy are typical of ball screw driven units.

Ball Guides

A ball guide consists of a ball rail and a ball bushing. The ball rail is made of hardened steel and runs along the inside of the profile. The ball bushing is attached to the carriage of the unit and contains balls that roll against the rail. The balls in the bushing can be recirculating or have fixed ball positions depending on the type of ball guide. The recirculating type has a longer life and better load capability while the fixed type typically is much smaller. Danaher Motion uses three major types of ball guides in its linear units. Either the compact single rail type with recirculating ball bushing (A), the stronger double rail type also with recirculating ball bushings (B) or the fixed ball position ball bushings type (not shown) which require very little space and are used in the smallest units. Ball guides offer high accuracy, high loads and medium speed.

Ball Screw Units with Double Ball Nuts

Using double ball nuts will increase the repeatability of the unit. The ball nuts are installed so that they are pre-tensioned against each other eliminating the play between the nuts and the screw. A double nut unit will have a slightly shorter stroke for a given overall length.

Single Carriage

Single carriage units have one carriage. Some linear unit models also have the option of long or short single carriage. The long carriage handle higher loads but will have a longer overall length for a given stroke.

Double Carriages

Double carriage units have two carriages which gives them higher load capabilites than single carriage units. When ordering a double carriage unit the distance between the two carriages needs to be defined. This distance is called LA or Lc depending on the model.

Units with Left/right Moving Carriages

Units with left/right moving carriages have two carriages moving in opposite directions when the drive shaft is rotated. This type of unit has a ball screw where half of the screw has a left hand thread and the other half a right hand thread.

3D CAD models are available at this link : www.tollo.com/3d_cad_drawings.htm